As geometries continue to shrink, manufacturers have increasingly turned to optical techniques such as ellipsometry and reflectometry to perform non-destructive inspection and analysis of semi-conductor wafers. Techniques of this type are commonly referred to as optical metrology and are based on the notion that a subject may be examined by analyzing the reflected energy that results when a probe beam is directed at the subject. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in magnitude are analyzed. Ellipsometry and reflectometry are effective methods for measuring a wide range of attributes including information about thickness, crystallinity, composition and refractive index. The structural details of ellipsometers are more fully described in U.S. Pat. Nos. 5,910,842 and 5,798,837 both of which are incorporated in this document by reference.
Scatterometry is a related optical metrology technique that measures the diffraction (optical scattering) that results when a probe beam is directed at a subject. Scatterometry is an effective method for measuring the critical dimensions (CD) of structural features (such as the lines and other structures included in integrated circuits). Scatterometry can be used to analyze two periodic two-dimensional structures (e.g., line gratings) as well as periodic three-dimensional structures (e.g., patterns of vias or mesas in semiconductors). Scatterometry can also be used to perform overlay registration measurements. Overlay measurements attempt to measure the degree of alignment between successive lithographic mask layers.
As shown in FIG. 1, a typical optical metrology tool includes a light source that creates a probe beam. The probe beam is focused by one or more lenses on a subject. The subject reflects the probe beam. The reflected energy is focused by another lens (or lenses) before reaching a detector. A processor analyzes the measurements made by the detector. These basic components in different configurations and variations are common to most optical metrology tools including ellipsometers, reflectometers, and scatterometers. The basic similarity between these different systems often means that a single system will be configured to perform multiple types of analysis. For this reason, it is common to see metrology systems that include one or more illumination sources combined with a series of detectors. This can happen, for example when a single system includes detectors to perform both ellipsometry and reflectometry. Single systems may also include multiple detectors to perform different types of ellipsometry or reflectometry. Multiple detectors may also be used to separately analyze different spectral portions of a reflected probe beam. Each of these cases is illustrated by the metrology system described in U.S. Pat. Ser. No. 6,278,519 which is incorporated herein by reference. As described in the latter patent, a metrology system can include multiple measurement modalities with separate detectors. Additional detectors not illustrated in the latter patent are typically provided for monitoring incident light levels used for normalization and calibration.
The use of multiple detectors creates multiple data streams each of which must typically be converted to digital form prior to analysis. For this reason, most optical metrology systems include an analog to digital converter (A/D converter) and a selector or multiplexer that is used to select between the different detectors. Unfortunately, the use of a single A/D converter multiplexed between multiple detectors has known drawbacks. One such drawback is possibility that errors may occur in the analog transmissions between the detectors and A/D converter. These transmissions typically take place over cable links between the detectors and the A/D converter. The cables are generally susceptible to electrical noise from a range of sources and must be carefully shielded to reduce the potential for errors. Even when shielded, errors may still occur and are often difficult to detect or repair.
A second drawback becomes apparent when the output of multiple detectors must be sampled simultaneously. When a single A/D converter is used, this type of sampling is performed serially, one detector at a time. This introduces a time-skew into the data generated for different detectors. This can reduce the accuracy of calculations that are based on the outputs of multiple detectors.